Essential Depot, The Lye Guy, Bulk Apothecary, to name a few - all good quality. Check prices with shipping to your area, and go with the least expensive.
Or, pick up the phone book (white pages) or do a google search for chemical suppliers in your area and see if you can get some that way. Sometimes you have to have an FEIN or an EIN for them to sell to you, but before my source went out of business a few years ago I was able to get my lye for .50 a pound by getting it from them, with no shipping costs to add in, because I was able to pick it up.